1. Field of the Invention
This invention relates to improvements in an intake manifold for a multi-cylinder internal combustion engine, and more particularly to an arrangement of the intake manifold having a gas passage through which additional gas such as blow-by gas or EGR (Exhaust Gas Recirculation) gas is introduced into an intake system of the engine.
2. Description of the Prior Art
In general, an internal combustion engine is provided with an intake manifold which is connected to an intake air collector or to the downstream side of a carburetor and has branch runners corresponding to engine cylinders. Intake air is distributed through the intake manifold into the respective engine cylinders.
Blow-by gas or EGR gas for exhaust gas emission control is required to be mixed with the intake air to be supplied to the respective engine cylinders. For this purpose, usually an outer gallery formed of a metal pipe is provided separate from and along the length of a cylinder head. A plurality of branch pipes are branched off from the outer gallery so that the blow-by gas or the EGR gas is introduced into the upstream sides of the intake ports of the respective engine cylinders. This is disclosed for example in Japanese Utility Model Publication No. 55-40365. Otherwise, there is a case in which the blow-by gas or the EGR gas is introduced directly to the intake air collector and distributed through the intake manifold into the respective engine cylinders.
However, in the former case in which the blow-by gas or the like is distributed through the outer gallery, the number of parts around the intake manifold increases thereby raising the production cost of the engine while complicating the layout of parts around the cylinder head. In addition, since the branch pipes for the respective engine cylinders extend from the outer gallery formed of one pipe, distribution of the gas into the respective engine cylinders tend to become not uniform according to engine operating conditions.
In the latter case in which the gas is introduced directly to the intake air collector, the location to which the gas is introduced is near the throttle valve and therefore there arises a problem that the throttle valve tends to be contaminated with contaminant in the gas under backward flow of the intake air to the carburetor. Additionally, if the intake air collector is located below the outlets of the intake manifold, condensed water from the EGR gas or the like is unavoidably gathered in the intake air collector. Thus, such a type of gas distribution arrangement is not practical.